The subject of the present invention is a device for controlling or monitoring the start of introduction, under pressure, of a fuel-air mixture following scavenging of an engine cylinder by air. Thus, the present invention relates to monitoring the start of pneumatic injection. The pressure source employed is supplied by the pressure prevailing in the crankcase which is offset at an angle and can be, in particular, retarded 120.degree. (in the case of an engine with 3, 6, . . . , 3n cylinders) or retarded 90.degree. (for an engine with 4, 8, . . . , 4n cylinders) relative to the cylinder in question into which the fuel-air mixture is introduced. This pressure source is not stored.
The present invention can be applied particularly to two-cycle internal combustion engines with spark ignition.